Modified Vaccinia Ankara (MVA) virus is related to vaccinia virus, a member of the genera Orthopoxvirus in the family of Poxyiridae. MVA was generated by 516 serial passages on chicken embryo fibroblasts of the Ankara strain of vaccinia virus (CVA) (for review see Mayr, A., et al. Infection 3, 6-14 [1975]). As a consequence of these long-term passages, about 31 kilobases of the genomic sequence were deleted from the virus and, therefore, the resulting MVA virus was described as being highly host cell restricted to avian cells (Meyer, H. et al., J. Gen. Virol. 72, 1031-1038 [1991]). It was shown in a variety of animal models that the resulting MVA was significantly avirulent (Mayr, A. & Danner, K. [1978] Dev. Biol. Stand. 41: 225-34). Additionally, this MVA strain has been tested in clinical trials as a vaccine to immunize against the human smallpox disease (Mayr et al., Zbl. Bakt. Hyg. I, Abt. Org. B 167, 375-390 [1987], Stickl et al., Dtsch. med. Wschr. 99, 2386-2392 [1974]). These studies involved over 120,000 humans, including high-risk patients, and proved that compared to vaccinia based vaccines, MVA had diminished virulence or infectiousness while it induced a good specific immune response.
In the following decades, MVA was engineered for use as a viral vector for recombinant gene expression or as a recombinant vaccine (Sutter, G. et al. [1994], Vaccine 12: 1032-40).
In this respect, it is significant that even though Mayr et al. demonstrated during the 1970s that MVA is highly attenuated and avirulent in humans and mammals, some recently reported observations (Blanchard et al., 1998, J Gen Virol 79, 1159-1167; Carroll & Moss, 1997, Virology 238, 198-211; Altenberger, U.S. Pat. No. 5,185,146; Ambrosini et al., 1999, J Neurosci Res 55(5), 569) have shown that MVA is not fully attenuated in mammalian and human cell lines since residual replication might occur in these cells. It is assumed that the results reported in these publications have been obtained with various known strains of MVA since the viruses used essentially differ in their properties, particularly in their growth behavior in various cell lines.
Growth behavior is recognized as an indicator for virus attenuation. Generally, a virus strain is regarded as attenuated if it has lost its capacity or only has reduced capacity to reproductively replicate in host cells. The above-mentioned observation, that MVA is not completely replication incompetent in human and mammalian cells, brings into question the absolute safety of known MVA as a human vaccine or a vector for recombinant vaccines.
Particularly for a vaccine, as well as for a recombinant vaccine, the balance between the efficacy and the safety of the vaccine vector virus is extremely important.
Most viral vaccines such as attenuated or recombinant viruses are manufactured from cell culture systems. The cells used for virus/vaccine production may be cell lines, i.e. cells that grow continuously in vitro, either as single-cell suspension culture in bioreactors or as a monolayer on a cell-support surface of tissue culture flasks or roller-bottles. Some examples for cell lines used for the production of viruses are: the human fetal lung cell-line MRC-5 used for the manufacture of polio viruses and the human fetal lung cell-line WI-38 used for the manufacture of measles virus, mumps virus and rubella virus (MMR II) (Merck Sharp & Dohme).
Not only cell lines but also primary animal cells are used for the manufacture of vaccines. An example of primary cells that are used for virus production are chicken embryo fibroblasts (CEF cells). These cells are used for the production of measles and Japanese encephalitis virus (Pasteur Merieux), mumps virus (manufactured by Provaccine), rabies virus (manufactured by Chiron Berhing GmbH & Co.), yellow fever virus (manufacture by Aprilvax), influenza virus (manufactured by Wyeth Labs and SmithKline & Beecham) and modified Vaccinia virus Ankara (MVA).
CEF cells are often used since many virus vaccines are made by attenuating the virulent disease-causing virus by serially passaging in CEF cells. Attenuated viruses, such as MVA are preferably not propagated on human cells since there is a concern that the viruses might become replication competent in cells of human origin. Viruses that have regained the ability to replicate in human cells represent a health risk if administered to humans, in particular if the individuals are immune compromised. For this reason, some attenuated viruses, such as MVA, are strictly manufactured from CEF cells, if intended for human use.
Moreover, CEF cells are used for those viruses that grow only on said cells. Examples of such viruses are avian viruses such as avipox viruses, canary pox virus, ALVAC, Fowl pox virus and NYVAC.
Cell lines and primary cells grown under in vitro culturing conditions require a special growth and maintenance medium that can support (I) cell replication in the logarithmic phase and (II) cell maintenance once the cells are no longer dividing, i.e., when the cells are in the stationary phase. The commonly used cell culture media comprise a rich salt solution containing vitamins, amino acids, essential trace elements and sugars. Growth hormones, enzymes and biologically active proteins required for supporting cell growth and maintenance are usually added as a supplement to the medium in the form of an animal blood derived serum product. Examples of animal blood derived serum products are fetal calf serum, chicken serum, horse serum and porcine serum. These sera are derived from fractionated blood, from which the red blood cells and the white blood cells have been removed. Primary cells, such as CEF cells are even more dependent on animal serum sources than cell lines. Thus, primary cells are usually cultivated in cell culture media comprising 5 to 10% serum, in most cases fetal calf serum (FCS).
The animal sera not only comprise factors that are required for the growth of cells, but also factors that are required for cells that naturally grow as adherent cells to attach to the cell support surface of the culture vessel. Thus, it is critical for adherent cells that enough serum is added to the medium to enable them to grow and form a monolayer.
Unfortunately, bovine/fetal calf serum as well as sera from other animals may contain adventitious pathogenic agents such as viruses or prion proteins. There is a potential risk that these pathogenic agents may be transmitted to the animal/human to be treated or vaccinated with the vaccine or any other pharmaceutical product produced in cell culture. This is of particular relevance if cell culture products are administered to immune-compromised humans. One of the many potential major problems associated with the commonly used bovine serum supplement is the possibility to transmit the agent causing bovine spongiforme encephalopathy (BSE) to the animals/humans that come into contact with the products produced from cell culture.
In view of the possible risk associated with the use of animal sera in cell culture it has become clear that manufacturing processes free from the use of animal products are highly desirable.
To this end, specific media that do not have to be supplemented with animal sera have been developed for continuously growing cell lines and for the production of viruses in continuously growing cell lines, respectively. An example of such a serum free medium that can be used to cultivate cell lines is VP-SFM manufactured by Gibco BRL/Life Technologies. According to the manufacturer's information VP-SFM is designed specifically for the growth of VERO, COS-7, MDCK, Hep2, BHK-21 and other important cell lines (Price, P. and Evege, E. Focus 1997, 19: 67-69) and for virus production in said cell lines. No information is available regarding the cultivation of primary cells in the medium.